ENH(UX): do not repeat entry for token and aws key/secret

[yarikoptic]

I failed to find an issue but I know that it annoyed many people, me included,
whenever long secrets such as tokens and keys for AWS were asked to be repeated.
There is IMHO no reason for that since they are never typed in and validation
of correct entry is beyond human capabilities without much straining

A followup to #5218

[kyleam]

Sounds good to me.

[codecov]

Codecov Report

Merging #5219 (4aa19c4) into maint (1b013a8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            maint    #5219      +/-   ##
==========================================
- Coverage   89.91%   89.89%   -0.02%     
==========================================
  Files         294      294              
  Lines       40960    40963       +3     
==========================================
- Hits        36829    36824       -5     
- Misses       4131     4139       +8     

Impacted Files	Coverage Δ
datalad/downloaders/credentials.py	82.35% <100.00%> (+0.31%)	⬆️
datalad/downloaders/tests/test_http.py	88.59% <0.00%> (-1.91%)	⬇️

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